It's about context.
If this was a mostly-white school and there was an offensive picture of a homeless white guy or a white crack whore, that would be racist.
Sorry, don't agree. A homeless guy is a homeless guy. A crack whore is a crack whore. Injecting color into it is irrelevant whether it is a mostly white school or a black school... or at least it should be unless we are going to dive deep into the prejudice of lowered expectations.